netfilter: nft_hash: validate maximum value of u32 netlink hash attribute

Use the function nft_parse_u32_check() to fetch the value and validate
the u32 attribute into the hash len u8 field.

This patch revisits 4da449ae ("netfilter: nft_exthdr: Add size check
on u8 nft_exthdr attributes").

Fixes: cb1b69b0 ("netfilter: nf_tables: add hash expression")
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>